Process for preparing an aqueous powder coating dispersion and using the same

ABSTRACT

The present invention relates to an aqueous powder coating dispersion comprising a solid, pulverulent component A and an aqueous component B, in which component A. is a powder coating comprising 
     a) at least one epoxide-containing binder with a content of from 30 to 45%, preferably from 30 to 35%, of glycidyl-containing monomers and, if desired, with a content of aromatic vinyl compounds, preferably styrene, 
     b) at least one crosslinking agent, preferably straight-chain aliphatic dicarboxylic acids and/or carboxy-functional polyesters, and 
     c) if desired, catalysts, auxiliaries, typical powder coating additives, such as degassing agents, levelling agents, UV absorbers, free-radical scavengers and antioxidants, 
     and component B. is an aqueous dispersion comprising 
     a) at least one nonionic thickener and 
     b) if desired, catalysts, auxiliaries, defoaming agents, dispersion auxiliaries, wetting agents, preferably carboxy-functional dispersants, antioxidants, UV absorbers, free-radical scavengers, biocides, small quantities of solvent and/or hygroscopic agents.

The application also relates to a process for the preparation of thepowder coating dispersion and to its use for car bodies.

BACKGROUND OF THE INVENTION

The present invention relates to an aqueous powder coating dispersionwhich is particularly suitable as a coating for car bodies which havebeen coated with aqueous basecoat.

For the coating of car bodies, preference is currently given to the useof liquid coating materials. These cause numerous environmental problemsowing to their solvent content. The same applies to cases wherewater-based coating materials are employed.

For this reason, increased efforts have been made in recent years to usepowder coatings for the coating operation. The results so far, however,are not satisfactory; in particular, increased coat thicknesses arenecessary in order to achieve a uniform appearance. On the other hand,the use of pulverulent coating materials entails a different applicationtechnology. Plants designed for liquid coating materials, therefore,cannot be used for the powders. The aim is therefore to develop powdercoatings in the form of aqueous dispersions which can be processed withliquid coating technologies.

U.S. Pat. No. 4,268,542, for example, discloses a process in which apowder coating slurry is used which is suitable for the coating of cars.In this process, a conventional powder coat is first applied to thebody, and the clearcoat slurry is applied as second coat. In thisclearcoat slurry, which is based on acrylate resins, ionic thickenersare used. Furthermore, in one of the examples these thickeners containfrom 0.5 to 30% of glycidyl-containing monomers. Moreover, high stovingtemperatures (above 160° C.) are necessary.

The present invention has now set itself the object of providing anaqueous powder coating dispersion which can be applied to car bodies bythe existing liquid coating technology and which, in particular, can bestoved even at temperatures of 130° C.

SUMMARY OF THE INVENTION

This object is achieved in that the aqueous powder coating dispersioncan be prepared by subjecting an aqueous dispersion of a powder coatinghaving a glass transition temperature of from 20 to 90° C., preferablyfrom 40 to 70° C., a viscosity of from 10 to 1000 mPas, preferably from50 to 300 mPas, at a shear rate of 500 s⁻¹ and a solids content of from10 to 50%, preferably from 20 to 40% by weight, to a grinding processwhile maintaining a temperature of from 0 to 60° C., preferably from 5to 35° C. The specific energy input during the grinding process ispreferably from 20 to 500 Wh/kg, in particular from 50 to 250 Wh/kg.

DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 provide an illustration of a grinding process for use inpreparing the instant invention.

DETAILED DESCRIPTION OF THE INVENTION

An aqueous powder coating dispersion employed in accordance with theinvention comprises a solid, pulverulent component A and an aqueouscomponent B, in which

component A. is a powder coating comprising

a) at least one epoxide-containing binder with a content of from 30 to45% by weight, preferably from 30 to 35%, of glycidyl-containingmonomers and, if desired, with a content of aromatic vinyl compounds,preferably styrene,

b) at least one crosslinking agent, preferably straight-chain aliphaticdicarboxylic acids and/or carboxy-functional polyesters, and

c) if desired, catalysts, auxiliaries, typical powder coating additives,such as degassing agents, levelling agents, UV absorbers, free-radicalscavengers and antioxidants,

and

component B. is an aqueous dispersion comprising

a) at least one nonionic thickener and

b) if desired, catalysts, auxiliaries, defoaming agents, dispersionauxiliaries, wetting agents, preferably carboxy-functional dispersants,antioxidants, UV absorbers, free-radical scavengers, small quantities ofsolvent, antioxidants, UV absorbers, free-radical scavengers, smallquantities of solvent, levelling agents, biocides and/or hygroscopicagents.

Dispersions with such a composition are used in particular for powderclearcoats.

Suitable epoxy-functional binders for the solid powder coating used toprepare the dispersion are, for example, polyacrylate resins whichcontain epoxide groups and which can be prepared by copolymerization ofat least one ethylenically unsaturated monomer containing at least oneepoxide group in the molecule with at least one other ethylenicallyunsaturated monomer containing no epoxide group in the molecule, atleast one of the monomers being an ester of acrylic acid or methacrylicacid. Epoxide group-containing polyacrylate resins of this kind areknown, for example, from EP-A-299 420, DE-B-22 14 650, DE-B-27 49 576,U.S. Pat. No. 4,091,048 and U.S. Pat. No. 3,781,379).

Examples of ethylenically unsaturated monomers containing no epoxidegroup in the molecule are alkyl esters of acrylic and methacrylic acidcontaining 1 to 20 carbon atoms in the alkyl radical, especially methylacrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butylacrylate, butyl methacrylate, 2-ethylhexyl acrylate and 2-ethylhexylmethacrylate. Other examples of ethylenically unsaturated monomerscontaining no epoxide groups in the molecule are acid amides, forexample acrylamide and methacrylamide, aromatic vinyl compounds, such asstyrene, methylstyrene and vinyltoluene, nitriles, such as acrylonitrileand methacrylonitrile, vinyl halides and vinylidene halides, such asvinyl chloride and vinylidene fluoride, vinyl esters, for example vinylacetate, and hydroxyl-containing monomers, for example hydroxyethylacrylate and hydroxyethyl methacrylate.

The polyacrylate resin containing epoxide groups usually has an epoxideequivalent weight of from 400 to 2500, preferably from 420 to 700, anumber-average molecular weight (determined by gel permeationchromatography using a polystyrene standard) of from 2000 to 20,000,preferably from 3000 to 10,000, and a glass transition temperature(T_(G)) of from 30 to 80° C., preferably from 40 to 70° C., particularlypreferably from 40 to 60° C. (measured by means of Differential ScanningCalorimetry (DSC)). A temperature of about 50° C. is very particularlypreferred. It is also possible to employ mixtures of two or moreacrylate resins.

The polyacrylate resin containing epoxide groups can be prepared inaccordance with generally well-known methods by polymerization.

Suitable crosslinking agents are carboxylic acids, especially saturated,straight-chain aliphatic dicarboxylic acids having 3 to 20 carbon atomsin the molecule. Very particular preference is given to the use ofdecane-1,12-dicarboxylic acid. To modify the properties of the finishedpowder clearcoats it is also possible, if desired, to employ othercarboxyl-containing crosslinking agents. Examples hereof which may bementioned are saturated branched or unsaturated straight-chain di- andpolycarboxylic acids, and also polymers containing carboxyl groups.

Also suitable are powder coatings comprising an epoxy-functionalcrosslinking agent and an acid-functional binder.

Examples of suitable acid-functional binders are acidic polyacrylateresins which can be prepared by copolymerizing at least oneethylenically unsaturated monomer containing at least one acid group inthe molecule with at least one other ethylenically unsaturated monomercontaining no acid group in the molecule.

The binder containing epoxide groups or the crosslinking agentcontaining epoxide groups and the carboxyl or, respectively, the binderare usually employed in a quantity such that there are from 0.5 to 1.5,preferably from 0.75 to 1.25, equivalents of carboxyl groups perequivalent of epoxide groups. The quantity of carboxyl groups presentcan be determined by titration with an alcoholic KOH solution.

In accordance with the invention the binder comprises aromatic vinylcompounds, especially styrene. In order to limit the danger of cracking,however, the content is not more than 35% by weight. From 10 to 25% byweight is preferred.

The solid powder coatings comprise, if desired, one or more suitablecatalysts for the epoxy resin curing. Suitable catalysts are phosphoniumsalts of organic or inorganic acids, quaternary ammonium compounds,amines, imidazole and imidazole derivatives. The catalysts are ingeneral employed in proportions of from 0.001% by weight to about 2% byweight, based on the overall weight of the epoxy resin and of thecrosslinking agent.

Examples of suitable phosphonium catalysts are ethyltriphenylphosphonium iodide, ethyltriphenylphosphonium chloride,ethyltriphenylphosphonium thiocyanate, ethyltriphenylphosphoniumacetate/acetic acid complex, tetrabutylphosphonium iodide,tetrabutylphosphonium bromide and tetrabutylphosphonium acetate/aceticacid complex. These and other suitable phosphonium catalysts aredescribed, for example, in U.S. Pat. No. 3,477,990 and U.S. Pat. No.3,341,580.

Examples of suitable imidazole catalysts are 2-styrylimidazole,1-benzyl-2-methylimidazole, 2-methylimidazole and 2-butylimidazole.These and other imidazole catalysts are described, for example, inBelgian Patent No. 756,693.

The solid powder coatings can additionally, if desired, compriseauxiliaries and additives as well. Examples of these are levellingagents, antioxidants, UV absorbers, free-radical scavengers, flow aidsand degassing agents such as, for example, benzoin.

Suitable levelling agents are those based on polyacrylates,polysiloxanes and/or fluorine compounds.

Antioxidants which can be employed are reducing agents, such ashydrazides and phosphorus compounds, and also free-radical scavengers,for example 2,6 di-tert-buthylphenol derivatives.

UV absorbers which can be used are preferably triazines andbenzotriphenol.

As free-radical scavengers which can be employed, preference is given to2,2,6,6 tetramethylpiperidine derivatives.

As a further constituent the aqueous component B of the powder coatingdispersion contains at least one nonionic thickener a). Preference isgiven to the use of nonionic associative thickeners a). Structuralfeatures of such associative thickeners a) are:

aa) a hydrophilic structure which ensures adequate solubility in water,and

ab) hydrophobic groups which are capable of associative interaction inthe aqueous medium.

Examples of hydrophobic groups employed are long-chain alkyl radicals,for example dodecyl, hexadecyl or octadecyl radicals, or alkarylradicals, for example octylphenyl or nonylphenyl radicals.

Hydrophilic structures employed are preferably polyacrylates, celluloseethers or, with particular preference, polyurethanes, which comprise thehydrophobic groups as polymer units.

As hydrophilic structures, very particular preference is given to theuse of polyurethanes comprising polyether chains as structural units,preferably comprising polyethylene oxide. In the synthesis of suchpolyetherpolyurethanes, the di- and/or polyisocyanates, preferablyaliphatic diisocyanates, particularly preferably unsubstituted oralkyl-substituted 1,6-hexamethylene diisocyanate, serve for the linkingof the hydroxyl-terminated polyether units with one another and for thelinking of the polyether units with the hydrophobic end group units,which may for example be monofunctional alcohols and/or amines havingthe long-chain alkyl radicals or aralkyl radicals already mentioned.

Component B can additionally comprise catalysts, levelling agents,antioxidants, UV absorbers, free-radical scavengers and wetting agents.Suitable substances in this context are essentially those already listedfor component A.

Further agents which can be added to component B are auxiliaries,defoaming agents, dispersion auxiliaries, biocides, solvents andneutralizing agents.

Suitable defoaming agents are preferably modified polysiloxanes.

Preferred examples of dispersion auxiliaries are ammonium or metal saltsof polycarboxylates.

Neutralizing agents which can be used are amines, ammonia and metalhydroxides.

The solid powder coatings are prepared by known methods (cf. e.g.product information sheet "Pulverlacke" [Powder coatings] from BASFLacke +Farben AG, 1990) by homogenization and dispersion, for example bymeans of an extruder, screw-type compounding unit, and the like. Afterthe powder coatings have been prepared, they are prepared for dispersionby grinding and, if appropriate, by classifying and screening.

The aqueous powder clearcoat dispersion can subsequently be preparedfrom the dry-ground powder coating by wet grinding or by introductionwith stirring. Wet grinding is particularly preferred.

Accordingly, the present invention also relates to a process for thepreparation of an aqueous powder coating dispersion, in the course ofwhich this dispersion of a powder coating having a glass transitiontemperature of from 20 to 90° C., preferably from 40 to 70° C., and aviscosity of from 10 to 1000 mPas, preferably from 50 to 300 mPas, isprepared at a shear rate of 500 s⁻¹ and a solids content of from 10 to50% by weight, preferably from 20 to 40%, this dispersion is groundwhile maintaining a temperature of from 0 to 60° C., preferably from 5to 35° C., a pH of from 4.0 to 7.0, preferably from 5.5 to 6.5, isestablished and the dispersion is filtered.

The specific energy input during the grinding process is preferably from20 to 500 Wh/kg.

In a preferred embodiment of the invention, the process for thepreparation of an aqueous powder coating dispersion relies on the basisof the component A described, which in accordance with the invention isdispersed in a component B. The latter comprises an aqueous dispersionof at least one nonionic thickener and, if desired, of catalysts,auxiliaries, antifoam agents, antioxidants, wetting agents, UVabsorbers, free-radical scavengers, biocides, hygroscopic agents, smallquantities of solvents and/or dispersion auxiliaries, preferablycarboxy-functional dispersion auxiliaries.

In accordance with the invention, grinding is carried out after thedispersion of component A in component B.

The aqueous powder coating dispersion may comprise a solvent content offrom 0 to 10% by weight, based on the total weight of the aqueous powdercoating dispersion.

The mean particle size obtained is between 1 and 25 μm, preferably below20 μm. With particular preference it is from 3 to 10 μm.

It is an essential feature of the invention that during the grindingprocess the dispersion contains only small quantities of solvent. It maytherefore be necessary under certain circumstances to free the grindingapparatus from solvent residues before beginning the grinding process.

Before or after wet grinding and/or the introduction of the dry powdercoating into the water, it is possible for from 0 to 5% by weight of adefoamer mixture, an ammonium and/or alkali metal salt, acarboxyl-functional or nonionic dispersion auxiliary, a wetting agentand/or a thickener mixture, and the other additives, to be added to thedispersion.

In accordance with the invention, defoamers, dispersion auxiliaries,wetting agents and/or thickeners are first of all dispersed in water.Then small portions of the powder clearcoat are stirred in.Subsequently, defoamers, dispersion auxiliaries, thickeners and wettingagents are again incorporated by dispersion. Finally, powder clearcoatsare again stirred in in small portions.

In accordance with the invention, the pH is preferably established usingammonia or amines. In this context the pH may initially rise, so that astrongly basic dispersion is produced. However, the pH falls back to thevalues indicated above within a period of several hours or days.

The powder coating dispersion according to the invention can be used inparticular in the form of a clearcoat as a coating over basecoats,preferably in the automotive industry. Such a clearcoat dispersion isparticularly suitable for aqueous basecoats based on a polyester, apolyurethane resin and an amino resin.

The powder coating dispersions according to the invention can be appliedby methods known from liquid coating technology. They can in particularbe applied by spray techniques. Also suitable are electrostaticallyassisted high-speed rotation or pneumatic application.

The powder clearcoat dispersions applied to the basecoat are in generalflashed off before stoving. This is expediently carried out first atroom temperature and then at slightly elevated temperature. In generalthe elevated temperature is from 40 to 70° C., preferably from 50 to 65°C. The flash-off time is from 2 to 10 minutes, preferably from 4 to 8minutes at room temperature. At elevated temperature, flashing off isrepeated for the same period of time.

Stoving can be carried out at temperatures from as low as 130° C. It ispossible to carry out stoving at from 130 to 180° C., preferably from135 to 155° C.

Using the process according to the invention it is possible to achievecoat thicknesses of from 30 to 50 μm, preferably from 35 to 45 μm.Clearcoats of comparable quality could hitherto, in accordance with theprior art, be achieved, with the use of powder clearcoats, only byapplying coat thicknesses of from 65 to 80 μm.

Example of the production of a powder slurry

The equipment used to produce the powder slurry and the procedure aredepicted diagrammatically in FIGS. 1 and 2.

The production process is as follows:

1. Weighing-in of the liquid components

Deionized water, thickeners, wetting agents and dispersants areintroduced over a dissolver D containing a motor unit M in the containerB1. After adding the powder to the liquids, the batch is subjected tothe action of the dissolver for 20 minutes (peripheral disc PD speed 20m/s).

2. Initial flushing of the stirred ball mill

Before the beginning of wet grinding, the mill MI is flushed with amixture of deionized water and the additives contained in the productuntil the machine is free from other solvents.

3. Wet grinding

The product batch described in 1. is ground with the aid of a stirredball mill MI. For this purpose the product is supplied to the mill MIthrough a gauge G1 by means of a pump PU1 and is circulated around themill MI until the desired end quality (x₅₀ <4μ, x_(max) <10μ) is reachedwith a specific energy input of about 60 Wh/kg. The three-way valve G2is positioned accordingly. The maximum temperature reached is less than25° C. Thereafter, the powder slurry is discharged by way of transfermeans R1 into a vessel B2, the three-way valve G2 being positionedaccordingly.

4. Completion of the powder slurry

After wet grinding, the powder slurry is made up. This is done by addingadditional small quantities of additives (thickeners, wetting agents,amine) with stirring.

5. Filtration

Filtration is carried out as shown in FIG. 2. The powder slurry of B2 isinitially filtered in circulation by way of pump PU2 with bag filters(PONG 50) in the filtration unit F comprising gauge P1. Filtration issubsequently carried out in one pass over the same bag filters in thefiltration unit F into a clean container B3. The slurry is thendispensed into its final containers.

What is claimed is:
 1. An aqueous powder coating dispersion, comprisingthe result ofdispersing a solid pulverulent component A into an aqueouscomponent B to provide an aqueous dispersion of a powder coatingcomposition having a glass transition temperature of from 20 to 90° C.,a viscosity of from 10 to 1000 mPas at a shear rate of 500 s⁻¹, and asolids content of from 20 to 40% by weight, subjecting said aqueousdispersion of a powder coating to a grinding process while maintaining atemperature of from 0 to 60° C. to provide an aqueous powder coatingdispersion having a solids content of from 20 to 40% by weight, whereinsolid pulverulent component A is a powder coating compositioncomprisinga) at least one epoxide-containing binder comprising from 30to 45% by weight of glycidyl-containing monomers, b) at least onecrosslinking agent, and component B is an aqueous dispersion comprisinga) at least one nonionic thickener.
 2. The aqueous powder coatingdispersion of claim 1, wherein the grinding process has a specificenergy input between 20 and 500 Wh/kg.
 3. An aqueous powder coatingdispersion comprisingincorporating a first portion of the ground aqueousdispersion of a powder coating of claim 1 into an aqueous dispersioncomprising one or more additives selected from the group consisting ofdefoamers, dispersion auxiliaries, wetting agents, thickeners, andmixtures thereof, to create an aqueous dispersion containing the firstportion of ground aqueous dispersion of a powder coating, incorporatingone or more additional additives selected from the group consisting ofdefoamers, dispersion auxiliaries, wetting agents, thickeners, andmixtures thereof, into the aqueous dispersion containing the firstportion of ground aqueous dispersion of a powder coating, andincorporating a second portion of the ground aqueous dispersion of apowder coating of claim 1 into the aqueous dispersion containing thefirst portion of ground aqueous dispersion of a powder coating and oneor more additional additives to an aqueous dispersion of a powdercoating.
 4. The aqueous powder coating dispersion of claim 1 which is apowder clearcoat dispersion.
 5. The aqueous powder coating dispersion ofclaim 1 having a pH between 4.0 and 7.0.
 6. The aqueous powder coatingdispersion of claim 1 wherein the epoxide-containing containing binder Aa) further comprises aromatic vinyl compounds.
 7. The aqueous powdercoating dispersion of claim 1 wherein the epoxide-functional binderscomprise polyacrylate resins which contain epoxide groups, and theglycidyl-containing monomers are selected from the group consisting ofglycidyl acrylate, glycidyl methacrylate, allyl glycidyl esters andmixtures thereof.
 8. The aqueous powder coating dispersion of claim 1wherein nonionic thickener a) of component B comprises at least onenonionic associative thickener comprising:aa) a hydrophilic structureand ab) hydrophobic groups capable of associative interaction in anaqueous medium.
 9. The aqueous powder coating dispersion according toclaim 8, wherein nonionic associative thickener a) comprisespolyurethane chains as hydrophilic structure aa).
 10. The aqueous powdercoating dispersion according to claim 9, wherein nonionic associativethickener a) comprises polyurethane chains with polyether units ashydrophilic structure aa).
 11. The aqueous powder coating dispersion ofclaim 1, having a particle size not more than 20 μm.
 12. A process forthe preparation of an aqueous powder coating dispersioncomprisingpreparing an aqueous dispersion of a powder coatingcomposition by dispersing a solid pulverulent component A into anaqueous component B, the resulting aqueous dispersion having a glasstransition temperature of from 20 to 90° C., a viscosity of from 10 to1000 mPas at a shear rate of 500 s⁻¹, and a solids content of from 20 to40% by weight, and grinding the aqueous dispersion of a powder coatingcomposition while maintaining a temperature of from 0 to 60° C. toprovide a ground aqueous dispersion of a powder coating, establishing apH of from 4.0 to 7.0 in the ground aqueous dispersion of a powdercoating, and filtering the ground aqueous dispersion of a powder coatingdispersion to provide an aqueous powder coating dispersion, whereinsolid pulverulent component A is a powder coating compositioncomprisinga) at least one epoxide-containing binder comprising from 30to 45% by weight of glycidyl-containing monomers, b) at least onecrosslinking agent selected from the group consisting of straight chainaliphatic dicarboxylic acids, carboxy-functional polyesters, andmixtures thereof, and component B is an aqueous dispersion comprising a)at least one nonionic thickener.
 13. The process of claim 12, furthercomprising grinding the aqueous dispersion of a powder coatingcomposition in a grinding apparatus.
 14. The aqueous powder coatingdispersion of claim 1 wherein the aqueous dispersion of a powder coatingcomposition subjected to grinding has a glass transition temperature offrom 40 to 70° C., a viscosity of from 50 to 300 mPas at a shear rate of500 s⁻¹ and a solids content of from 20 to 40% by weight and thegrinding process is maintained at a temperature of from 5 to 35° C. 15.The aqueous powder coating dispersion of claim 1, wherein the grindingprocess has a specific energy input between 50 to 250 Wh/kg.
 16. Theaqueous powder dispersion of claim 1 wherein epoxide containing binderAa) comprises from 30 to 35% by weight of glycidyl-containing monomers.17. The aqueous powder dispersion of claim 1 wherein crosslinking agentAb) is selected from the group consisting of straight chain aliphaticdicarboxylic acids, carboxy functional polyesters, and mixtures thereof.18. The aqueous powder dispersion of claim 5 having a pH between 5.5 and6.5.
 19. The aqueous powder coating dispersion of claim 6 wherein theepoxide containing binder Aa) comprises no more than 35% by weight ofaromatic vinyl compounds based on the weight of component Aa).
 20. Theaqueous powder coating dispersion of claim 19 wherein the epoxidecontaining binder Aa) comprises 10-25% by weight of aromatic vinylcompounds based on the weight of component Aa).
 21. The aqueous powdercoating dispersion of claim 6 wherein the epoxide containing binder Aa)further comprises styrene.
 22. A process of coating an article with anaqueous dispersion of a powder coating composition, comprisingprovidingan aqueous powder coating dispersion comprising the result ofsubjectingan aqueous dispersion of a powder coating composition comprising atleast one crosslinking agent, the aqueous dispersion having a glasstransition temperature of from 20 to 90° C., a viscosity of from 10 to1000 mPas, and a solids content of from 20 to 40% by weight, to agrinding process while maintaining a temperature of from 0 to 60° C. ,and applying the aqueous dispersion of a powder coating composition toan article by means of electrostatically assisted rotation or pneumaticapplication.
 23. The process of claim 22 wherein the article is anautomotive car body or a component thereof.